1. Field of the Invention
The present invention relates to a fuser apparatus, an image forming apparatus including the fuser apparatus, and a fuser controlling method.
2. Description of the Related Art
Since the processing speeds of image forming apparatuses have been increased in recent years, it is imperative to improve the efficiency and service life of a fuser apparatus (fuser).
Such an image forming apparatus is generally supplied with electric power from a receptacle to which the apparatus is plugged in an office or the like. The receptacle has a limited power supply capacity. In Japan, for example, the receptacle has a rated voltage of 100V and a rated current of 15A. Therefore, the image forming apparatus should be designed so as to be operative under limited power supply conditions. On the other hand, the image forming apparatus is often required to have systematic and multifunctional capabilities and, therefore, includes peripheral devices such as an automatic document feeder (ADF), a finisher for stapling and punching, and a large capacity sheet cassette (LCC) provided in a main body thereof. These peripheral devices are also supplied with electric power from the same receptacle for their operations. Therefore, the fuser should be efficiently controlled for fixing a toner on a recording medium with the limited power supply.
Further, components of the fuser are liable to be degraded during use in a higher temperature environment and, therefore, replaced after their service lives. However, it is impermissible to shorten the cycle of the replacement of the components for increasing the processing speed of the image forming apparatus. Users demand to reduce downtime for the replacement of the components as well as to improve the efficiency of a high speed image forming apparatus. Further, the users demand to prolong the service lives of the components for reduction of maintenance costs. To meet these demands, it is necessary to improve materials for the components as well as to develop a fuser controlling method which suppresses the degradation of the components and prolongs the service lives of the components.
The fuser to be provided in the high speed image forming apparatus shares the power supply with the peripheral devices and, therefore, suffers from limitations of an office or domestic supply voltage and a breaker current capacity. The fuser needs a highly efficient heating method, i.e., a method for efficiently distributing power to a plurality of heat sources and quickly heating a heat roller and a press roller to predetermined temperatures.
In a conventional heat source energization controlling method, the power supply is steadily turned on (i.e., the power supply duty cycle is kept at 100%) until the surface temperatures of the heat roller and/or the press roller reach the predetermined fixing temperatures after a printing process is started. When the predetermined fixing temperatures are reached, the power supply is turned off (i.e., the power supply duty cycle is set at 0%). In this controlling method, the heat roller suffers from significant temperature overshoot in a standby state immediately after the end of the printing process. This reduces the service life of the heater roller (fuser roller). That is, the temperature overshoot significantly thermally deteriorates the roller, adversely affecting the durability of the roller.
One approach to the improvement of the efficiency of the fuser and the prolongation of the service lives of the components of the fuser is to suppress the temperature overshoot. A fuser member such as the heat roller for fixing the toner on the recording medium is controlled to be kept at the predetermined temperature. However, the temperature of the fuser member is actually fluctuated around a target temperature when the fuser member is heated by turning on and off a heater. That is, so-called “ripples” occur. Further, when recording media are successively transported to the fuser member, the recording media remove heat from the fuser member to reduce the temperature of the fuser member. Even if the heater is turned on upon detection of the temperature reduction of the fuser member, the fuser member suffers from a response delay because it has a heat capacity. Therefore, the temperature reduction continues for a while after the turn-on of the heater, and then the temperature of the fuser member starts increasing. The target temperature for the temperature control should be set at a level higher than a lower limit temperature at which the toner can be fixed after the temperature reduction.
During the fixation on the successively transported recording media, the heating of the heater is controlled so that the heat amount removed by the recording media and the heat amount supplied from the heater are balanced. Thus, the temperature of the fuser member can be kept around the target temperature. After the last recording medium is transported, no heat is removed from the fuser member. However, the temperature of the fuser member continues to increase due to the response delay attributable to the heat capacity of the fuser member, resulting in the temperature overshoot.
The fuser member should withstand a temperature change between the lower limit fixing temperature after the start of the fixing operation and the overshoot peak temperature after the end of the fixing operation. The range of the temperature change is a temperature ripple range during the fixing operation. During the fixing operation, it is tough for the components of the fuser member to withstand the temperature change in the high temperature range.
For easier understanding, a specific example of the fuser will be described. A typical type of the fuser is a roller fixing type. The roller fixing type fuser includes a fuser roller, a press roller, a heater disposed in the fuser roller, and a temperature sensor which detects the temperature of the peripheral surface of the fuser roller. The fuser roller applies heat to a recording medium for heating and fusing toner to fix the toner on the recording medium. The press roller and the fuser roller cooperatively hold the recording medium therebetween to transport the recording medium. The heater heats the surface of the fuser roller through heat conduction from the inside. Usable as the fuser roller is a roller including a metal core coated with a fluorine-containing resin for easier release of the toner, or a roller including a metal core coated with a rubber so as to have an elastic surface for increasing a nip width with respect to the press roller. If the contact surface temperature of the fuser roller is excessively increased due to the temperature overshoot, deterioration of the components of the roller is accelerated. In the worst case, the rubber coating is separated from the metal core due to the high temperature.
For example, the control target temperature is 180° C., and the lower limit temperature is 155° C. The overshoot peak temperature is 195° C., and a difference between the lower limit temperature and the peak temperature is 40° C.
A heater controlling method is known which suppresses the temperature overshoot after the end of the fixing operation (see, for example, Japanese Unexamined Patent Publication No. HEI8(1996)-328425). In the heater controlling method, after the lower limit temperature of the fuser roller is detected in the fixing operation, the heater is controlled so that a total heat amount to be applied to the fuser roller in a period between the detection of the lower limit temperature and the end of the fixing operation is reduced to a level smaller than a total heat amount to be applied to the fuser roller in a period between the start of the fixing operation and the detection of the lower limit temperature.
In the heater controlling method disclosed in the aforementioned patent publication, the heater is kept on in a period during which the roller contact surface temperature is reduced after the fixing operation is started by the start of a copying process, and turned on and off with a predetermined duty ratio after the roller contact surface temperature starts increasing. The patent publication states that the duty ratio may be determined according to the type of paper sheets, the rotation speed of the rollers, an inter-sheet distance and an ambient temperature around the apparatus, but discloses nothing about a specific method for determining the duty ratio.
As described above, the prolongation of the service life of the fuser roller and the improvement of the efficiency of the fuser can be achieved by suppressing the temperature overshoot of the fuser roller. However, it is difficult to eliminate the delay in the response to the detection of the temperature attributable to the heat capacity of the fuser roller. For suppression of the response delay, the heater may be controlled on the basis of a change in the detection temperature. Thus, the temperature overshoot is expected to be suppressed.